YEMEN - Part 1 - The Prospects & Geology.

A source of high quality oil, Yemen raised its capacity from 388,000 b/d in mid-1998 to over 510,000 b/d in mid-2002. But the target of 550,000 b/d by end-2002 was not reached and, instead, output has fallen to 413,500 b/d.

The major oilfields in Yemen have begun to decline and the most promising areas have been explored. As a result, major oilfields require expensive EOR systems. Yemeni oil production is not likely to reach the 1m b/d figure projected in the 1980s by some of Sanaa's leaders. But there are a number of new small fields coming on stream (see Part 2).

The bulk of Yemen's oil production is exported (see Part 3). Domestic oil consumption is to remain limited to less than 100,000 b/d in the near future (see Downstream Trends of this week's Review).

President Ali Saleh is keen on seeing Yemen's hydrocarbon potentials developed and makes it a point of meeting executives from key foreign oil companies. He personally approves E&P deals (see who's who in Downstream Trends No. 26)

The Geology - The North-Western Sector: The north-western sector of Yemen is smaller than that of the south-east, with an area of 195,000 sq km and a population of about 15 million. The south-eastern sector has an area of about 333,000 sq km and a population of over 4.1 million. It was mainly the series of big oil discoveries in the south-eastern sector which caused a civil war from May to July in 1994.

There are six main sedimentary basin in Yemen: the Red Sea Basin in the west, the Yarin Basin in the north-west, the oil/gas rich Sab'atayn (Maarib-Shabwa) Basin of the shared between the north-western and south-eastern sectors of the country, the equally rich Sir-Say'un Basin in the south-east bordering with Saudi Arabia's part of Rub' Al Khali (empty quarter), the Qamar-Jeza Basin in the east of the country bordering with Saudi Arabia and Oman, and the Gulf of Aden Basin in the south.

The Tihama region of the north-western sector, a hot semi-desert strip about 65 km wide, separates the Red Sea from the rugged volcanic mountain ranges of the interior. Rising steeply to 5,000-12,000 feet, these highlands have deposits of rock salt, silver, gold, coal, copper and other minerals, plus a sedimentary basin of about 30,000 sq km between the ancient city of Maarib and Wadi Al Jawf areas.

Offshore along the Red Sea coast there are fairly wide areas of shallow waters, from the border with Saudi Arabia down to Bab El Mandeb. In tectonic terms, some of the areas within the Red Sea Basin are interesting parts of the Arabian Shield. A stable shelf with Mesozoic-Cenozoic cover stretches far into the interior, from a point near Hodeidah, and along the north-western part of the Gulf of Aden.

The tectonic relationship between the Owen Fracture Zone and the Gulf of Aden is important for the study of their sedimentations. The magnetic zone, or new oceanic crust, of the Sheba Ridge is relevant to their study, together with the axial portions in the Owen Fracture Zone and the Gulf of Aden which form the southern limits of the main hydrocarbon reservoirs of the Middle East.

Onshore prospects first became the subject of interest in the early 1950s, when a European geologist on a private visit to the ancient city of Maarib encountered an intriguing fault. He found the surface rocks to be of particular interest. These eventually were identified as Amran limestones.

Subsequent reporting to his company failed to arouse enthusiasm, however, partly because of hostile tribes in that area. The ruling Zaidi Shiite Imams of North Yemen had sealed their territory's interior from the outside world. The numerous tribes of the north-west were armed and suspicious of foreigners. An unsuccessful search for oil along the Red Sea at the time was another discouraging factor.

In 1963, Aramco geologists indicated that a large area of sandstone and siltstone outcrops overlying the crystalline Basement in the south-west of Saudi Arabia extended to northern Yemen. Three years later it was said they were part of the Wajid formation, of Paleozoic sandstone about 950 metres thick. In 1968 Aramco data based on surface and subsurface studies suggested the Wajid formation in Yemen was of the Permian age. In the 1970s the outcrops were defined as being Cambro-Ordovician in age. A UNESCO map for Yemen in 1985 showed them as Ordovician.

The source area for the Wajid is not clearly defined, with some geologists saying it to be from elevated eastern areas adjacent to the Hadhramout Arch of south-eastern Yemen. There was the theory that the source was the crystalline Basement block in north-western Yemen's interior.

An older Paleozoic sequence was found to be truncating along these features from the north and north-east to the south and south-west, with adjacent exposures in Ethiopia. But more recently it was said that a Cambro-Ordovician age for the Wajid outcrops was unlikely.